The present invention relates to the art of removing pit scrap which accumulates during the processing of steel. More particularly, the present invention is directed to a composition for and method of fluidization of accumulated pit scrap in ingot soaking pits.
As a first step in the generalized steelmaking process, iron-bearing raw materials (principally iron oxides) are reduced to molten iron or pig iron in a blast furnace using coke carbon as the reducing agent. In the process, the pig iron absorbs from about 3 to about 4.5% carbon. Since most modern carbon steel contains considerably less than 1.0% carbon, the excess carbon is removed by controlled oxidation of mixtures of molten pig iron and melted iron steel scrap in steelmaking furnaces to produce carbon steels of the desired carbon content.
Modern steelmaking processes produce molten or liquid steel. After the molten steel has attained the desired chemical composition, it is tapped or poured from the furnace into a ladle, from which the steel is teemed into tall, usually rectangular, molds. After being removed from the molds, the resulting ingots are typically reheated to a uniform temperature and rolled or forged into various shapes. This so-called semifinished steel is then subjected to various forms of mechanical treatment (hot and cold rolling, forging, extruding, drawing, and the like) to form the finished product.
Soaking pits are typically employed to bring steel ingots to a temperature suitable for treatment in a rolling mill. These pits are generally deep chambers or furnaces of various shapes, most commonly square or rectangular, designed to permit uniform heating of ingots to the desired temperature with a minimum of overheating of the surface of the ingot. The ingots are placed in an upright position in the pits through an opening in the top which is fitted with a removable cover.
Adjustments for lack of uniform temperature distribution are typically made by firing at a uniform rate for a period of time, followed by a period during which the pit is dampered and minimal fuel is admitted. During the latter period, the steel is "soaked" to equalize the temperture between ingots in the pit and between the surface and the interior of individual ingots. This practice, known as "firing and dampering", provides a product with a uniform temperature at high production rates in spite of any uneven heating during the firing stage.
There is a wide variation in soaking pit productivity, which is related to live pit-hole area. This is the area available in the pit on which ingots can be placed for heating. With proper loading, live pit-hole area amounts to approximately 35 to 40% in modern pits; a maximum of about 50% is the best that can presently be achieved. An average month's practice with modern soaking pits is heating about 30 to 100 tons of ingots per hour per 1000 square feet of live pit-hole area.
Pit scrap, a form of waste material derived principally from scale (iron oxides) which forms on the surface of the ingots, accumulates on the sides and bottom of the soaking pits during use, reducing their service life. Over time, the accumulation of pit scrap significantly reduces the live pit-hole area. Removal of the pit scrap through conventional means is difficult and requires a periodic downtime at some expense. While a routine dipping or scraping of pit scrap during operating periods may prolong the service life of a given pit, this technique is effective only when loose, fluid pit scrap is present.
With the use of ever-increasing firing rates in soaking pits, there is also a tendency towards an increase in oxygen level in the pit scrap. This increase in oxygen content leads to an elevation of from 200.degree. F. to 300.degree. F. in the theoretical melting points of the mixtures of FeO and Fe.sub.3 O.sub.4 typically found in pit scrap. These higher oxide mixture melting temperatures have made it extremely difficult to fluidize the pit scrap in soaking pits, even through intense firing of empty soaking pits.